If your computer has a someone else can gain access to it undetected.

If your computer has a rootkit, someone else can gain access to it undetected.
The term rootkit denotes a set of software tools typically malicious.
 The purpose of this software is to enable access to a computer or areas of its software that is not otherwise allowed.
It is one of the most difficult types of malware to find and remove.

These are very for instance for math pemdas parenthesis, exponents, multiplication, division, addition, subtraction without that i would never be able to remember what to do first with it i can remember.